1. Field of the Invention
The present invention relates to a focus monitoring method suitable for focus control of an exposure apparatus used in the manufacture of semiconductor elements, liquid crystal display elements, and the like, an exposure apparatus using this focus monitoring method, and a mask used in this focus monitoring method.
2. Description of the Related Art
Along with recent device micropatterning, it is becoming difficult to obtain a sufficiently large process margin such as an exposure latitude or depth of focus. To prevent a decrease in yield by effectively using a small process margin, a technique for monitoring exposure dose and focus more precisely is necessary.
According to a conventional method of managing focus, a quality controlling mask with rhombic marks 101 as shown in FIG. 1A is used. Exposure is performed while changing the focus value. A focus point where a pattern length Le of a rhombic mark 102, as shown in FIG. 1B, transferred onto a wafer becomes maximum is determined as the best focus. In this case, the relationship between the pattern length Le and defocus is as shown in FIG. 2 (see Jpn. Pat. Appln. KOKAI Publication No. 10-335208).
At the optimal focus point, even a very small portion of a rhombic mark is resolved. As defocus proceeds, the resolution for the small portion decreases. Therefore, the pattern length Le of the rhombic mark transferred onto the wafer becomes maximum at the best focus position, and has almost symmetric characteristics between positive and negative defocus directions. The rhombic mark can be applied to obtaining the best focus previous to the start of an exposure process in manufacture.
The method of this type has the following problems. Assume that the focus conditions for a manufacturing lot exposed under the same exposure conditions by using a rhombic mark are to be controlled. If merely the pattern length Le of the rhombic pattern after transfer is monitored, (1) the shift direction of the focus cannot be obtained, and (2) focus monitoring is adversely affected by variations in exposure dose.
As another focus monitoring method, a method of detecting a variation amount of the focus as a positional shift of the pattern without being adversely affected by the exposure dose has been proposed (see U.S. Pat. No. 5,300,786).
The sensitivity of focus detection by means of a mark according to this method largely depends on the shape of a light source or a coherence factor a More specifically, with an exposure condition of a comparatively low "sgr", a sufficiently high sensitivity may be obtained. However, with a comparatively large "sgr" condition, which is a generally used condition, or an annular illumination condition, a sufficiently high sensitivity cannot be obtained.
With the above method, when creating the mark, a phase shifting film must be formed. This increases the load in the mask formation. Although this method may be applied to a quality controlling mask, it is difficult to apply it to an actual device mask.
Therefore, a focus monitoring method has been sought for which can measure focus with projection optics at a high sensitivity and high precision without using a special mask for focus monitoring.
According to a first aspect of the present invention, there is provided a focus monitoring method of transferring a focus monitor pattern on a mask illuminated with an electromagnetic wave or electron beam onto an exposure target substrate by projection optics, and measuring a shift of the pattern on the substrate, thereby monitoring an effective focus, comprising:
preparing the mask on which the focus monitor pattern comprising at least two types of pattern groups is formed;
illuminating a pattern group A of the at least two pattern groups with illumination light while a barycenter of an illumination light source of an illumination optics is in an off-axis state;
illuminating at least a pattern group B of the at least two pattern groups with illumination light while the barycenter of the illumination light source is in an on-axis state; and
measuring a positional deviation between the pattern groups A and B transferred onto the substrate.
According to a second aspect of the invention, there is provided an exposure apparatus which transfers a first and a second pattern on a focus monitor mask illuminated with an electromagnetic wave or electron beam onto an exposure target substrate by projection optics, and measures the first and the second pattern on the substrate, thereby monitoring an effective focus, comprising:
illumination optics for illuminating the first pattern while a barycenter of an illumination light source is in an on-axis state, and
a component interposed in the illumination optics and configured to be able to illuminate the second pattern while the barycenter thereof is in an off-axis state.
According to a third aspect of the invention, there is provided an exposure mask comprising:
a transparent substrate with a first and a second major surface, the second major surface opposing an illumination light source;
a first and a second pattern configured to focus monitor and formed on the first major surface; and
a component which is arranged on or in the transparent substrate so as to shield the second pattern with respect to the illumination light source and which sets a barycenter of the illumination light source in an off-axis state.
According to a fourth aspect of the invention, there is provided an exposure mask which transfers a device pattern onto a wafer through projection optics, comprising:
a transparent substrate with a major surface;
the device pattern formed in a pattern region on the major surface of the transparent substrate;
a focus monitor pattern arranged in a region outside the pattern region on the major surface of the transparent substrate and formed from two types of patterns with different diffraction angles; and
a shielding portion which shields one of positive and negative diffracted light components in two opposing directions which pass through a pupil of the projection optics and are diffracted by one pattern of the focus monitor pattern which has a larger diffraction angle.
According to a fifth aspect of the invention, there is provided a focus monitoring method employed in transferring a device pattern onto a wafer through projection optics by using an exposure mask in which a device pattern is formed in a pattern region on one major surface of a transparent substrate, comprising:
arranging, in a region outside the pattern region on one major surface of the transparent substrate, a focus monitor pattern formed from two types of patterns with different diffraction angles;
when transferring the device pattern onto a sample, performing a process of shielding one of positive and negative diffracted light components which pass through a pupil of the projection optics and are diffracted by one pattern of the focus monitor pattern which has a larger diffraction angle;
causing a positional shift between the two types of patterns of the focus monitor pattern on the sample; and
detecting the positional shift as a defocus.